1. Field of the Invention.
The present invention relates to the manufacture of printed circuits from photographic films carrying an image of the conductive circuit instrument for measuring the clear area on a film containing a printed circuit layout.
2. Description of the Prior Art.
Printed circuit fabrication requires the plating of a thin layer of metal on various portions of the circuit. To insure product quality and save precious metals, it is desirable to accurately control thickness of the plating applied to the printed circuit.
A layout containing the conductor pattern which is to be plated is usually imprinted upon a photographic film media. Different types of film media are used depending upon the particular plating process. Common film media include silver halide and diazo films. By measuring from the layout design the area to be plated, it is possible to determine how much metal will be required for each printed circuit. The plating process can then be controlled with greater accuracy using this information.
Instruments which measure the total amount of clear area within a film pattern are commercially available. Such instruments typically utilize a light source and a detector positioned within an enclosed area. After calibration a film is positioned between the light source and the detector. The ratio of the light received by the detector through the film to that received through a totally open film of the same type if roughly proportional to the amount of clear area within the film.
The prior measuring instruments are relatively large in size since they must accommodate and simultaneously illuminate the entire film area. In addition, the manual calibration procedures which must be performed on these instruments in order to obtain a reading are difficult and time-consuming. Variations of the intensity of the illumination at varying points across the surface of the film results in inaccuracies in measurement which can not be eliminated by the initial calibration steps.
A simple film area computer which can be conveniently positioned within a laboratory or manufacturing plant is desired. This instrument should be easy to use, requiring a minimum of effort on the part of the operator. A self-calibrating film area computer is very desirable. In addition, the instrument should be capable of accurately measuring and displaying both the total amount of clear area and the total area within the film. The instrument should also be compatible with all types of films, including silver halides and diazos.